MUSEUM OF COMPARATIVE ZOOLOGY. 225 



tion. Thus, if I be not mistaken, a protovertebra immediately in front 

 of somite I. has been wholly converted into mesenchymatic tissue ; the 

 kernel of the succeeding protovertebra (somite I.) has given rise to a 

 considerable quantity of mesenchyme ; and the process has been mani- 

 fested, though to a less degree, even in succeeding somites. Further- 

 more, having established the continuity of splanchnopleure and somato- 

 pleure with the median and lateral peripheral layers respectively of the 

 protovertebrae, it seems to me the more probable that the former as well 

 as the latter may give rise to mesenchyme. I have, in fact, seen condi- 

 tions directly in front of the first nephrostome which indicated a very 

 extensive production of mesenchyme from the lateral plate in that 

 region. 



My reason for dwelling at so great length on the derivatives of the 

 peripheral layer of the protovertebra is, that this layer plays an impor- 

 tant part in forming certain accessory portions of the pronephric system. 

 I refer to the capsule of the pronephros. Already in the preceding 

 stage I noted the occurrence of a lateral fold of the somatic layer im- 

 mediately dorsal to the constriction between protovertebra) and lateral 

 plates (Fig. G). In the younger individuals of Stage IV. the fold covers 

 the dorsal surface of the pronephric pouch, and extends a short distance 

 down on its lateral surface (Figs. 18-21, fnd. cps.). In the older set of 

 embryos it has reached the somatopleure ventral to the pronephros, and 

 thus forms a complete investing capsule. 



In frontal sections the fundament of the capsule may be seen to 

 consist of a series of segmental outgrowths from the successive pro- 

 tovertebrge. Later, these segmentally arranged structures fuse into a 

 continuous enveloping sheet. 



Lateral to the pronephros the capsule presents in general a two-layered 

 condition, the result of its having been formed as a fold ; but on ascend- 

 ing to the level of the lower boundary of the somite, these two layers 

 separate (Plate V. Fig. 45) ; one passes beneath the protovertebra, cover- 

 ing the pronephros on its dorsal aspect ; the other is continuous with the 

 somatic layer of the protovertebra, forming a lateral sheath to the myo- 

 tome. These layers are present in the region both of the pronephros and 

 of the duct, but are seen in their simplest condition in the region of the 

 second nephrostome (Fig. 20) ; not merely because this is the middle ol 

 the pronephros, but also because the process is somewhat modified in the 

 protovertebra next in front of it (somite II.). Somite II. is one of those 

 in which a considerable portion of the kernel of the protovertebra is con- 

 verted into mesenchyme. For this reason the inner layer of the capsular 



15 



